This research study is being done to gain a better understanding about brain networks that may be involved in depression. The investigators plan to examine how these networks change after the brain is stimulated with "Transcranial Magnetic Stimulation" (TMS). TMS is a way of stimulating the brain in order to mildly activate or mildly suppress different brain areas, and is used to treat some forms of depression. It is hoped that this study will facilitate learning more about the structure and function of different brain areas and the ways that they are interconnected to form networks, both in depressed people and in people without depression. In this research study, the effects of TMS will be measured by obtaining "pictures" of the brain with "Magnetic Resonance Imaging" (MRI) and with "Positron Emission Tomography" (PET). More specifically, this will be accomplished with a combined MRI and PET scanner, which is capable of simultaneously obtaining both MRI and PET images of the brain. This scanning paradigm will allow the assessment of local metabolic changes resulting from TMS (with PET images) and brain network changes resulting from TMS (with fMRI). Changes resulting from TMS between 20 subjects with depression and 20 healthy volunteers will be calculated and will form the main outcome measure.